How does genetic engineering work?

It is a simple idea whereby we take new working genes, named wild types, and replace the faulty genes of some ones cells. This means the cells will then take up the new DNA and make a functional version of the protein which cures the disease. There are different methods of inputting the gene into the patients cells. One being fat globules withe the DNA inside and others being viruses. It is however difficult using these methods to get the new DNA into all the body's cells. A way to do this however is do it when the number of cells in the body is low such as development when the human is only a zygote.